The invention relates to printing on plastic and in particular to promotion of adhesion of printing inks to a plastic surface.
Surfaces of plastic products of all types, such as for example plastic shopping bags, place mats, and decals are printed for purposes of advertisement and decoration. However, common printing inks do not readily adhere to many plastics. Often a plastic surface that is to be printed must be treated prior to printing so that inks that are used to print the surface adhere properly to the surface. The treatment generally comprises coating the surface with a primer material that has a high affinity for both the plastic surface and the inks used to print the surface. The affinities of the primer for both the plastic and the ink bonds ink applied to the primer coated surface to the surface. Typically; the primer comprises or is mixed with volatile organic compounds potentially dangerous to human health that are released into the atmosphere during application of the primer to the surface. In order to protect people present in work areas where the primer is being used from exposure to the compounds, substantial resources have to be invested to properly ventilate the work areas and to monitor concentrations of the compounds in the air in the work areas.
U.S. Pat. No. 4,832,984 describes a water based system for coating plastics to receive inks, in which two layers of coating material are applied.
WO 99/54143 describes a water based coating system in which a mixture of two materials is used for coating the substrate prior to printing with liquid ink. One of the materials is a polyacid and the other is a polybase. The coating is between 2 and 30 g/m2 of substrate. A surfactant is indicated as being useful. However, no details of its use are given.
EP 0 474 278 describes water based coating of an imaging film for dry toner. The coating includes two component mixed together.
An aspect of some embodiments of the invention relates to a method of coating a surface utilizing a coating material comprising an aqueous mixture of a first component having an affinity for a plastic substrate (hereinafter a xe2x80x9cplastic bonding-agentxe2x80x9d) and a second component having an affinity for toner (hereinafter a xe2x80x9ctoner bonding-agentxe2x80x9d).
In accordance with some embodiments of the invention the coating, has a weight per square meter, when dry, of less than 1.5 grams.
In accordance with some embodiments of the invention, the second material is a neutralized acidic polymer.
In accordance with some embodiments of the invention a surfactant is added to the mixture, the surfactant having a w/w percentage of 0.5-2.5 percent of the aqueous mixture.
An aspect of some embodiments of the present invention relates to providing a primer, hereinafter referred to as a xe2x80x9cpollution free primerxe2x80x9d, for bonding a toner ink to a plastic surface that does not release organic compounds into the air.
An aspect of some embodiments of the present invention relates to providing a pollution free primer that adheres with sufficient forces to the plastic surface and the toner so that the toner can be applied to the surface using an offset process.
In exemplary embodiments of the invention the affinities of the first and second components for each other and for the plastic and the toner are such that the toner is efficiently transferred to the plastic from a blanket used in an offset printing process. Preferably the first and second materials form relatively strong bonds to each other at least when dried.
In some embodiments of the present invention, the plastic bonding-agent is polyethylenimine (PEI) and the toner bonding-agent is a dispersed phase of MP4990, which is an aqueous dispersion of copolymer of polyethylene and acrylic acid, manufactured by Michelman, or its salt. Optionally, the primer is mixed with a surfactant that enhances wetting of the plastic surface by the primer and promotes formation of a uniform coating of the primer on the plastic surface.
The inventors have found that a thin substantially uniform coating of the primer can be applied to a side of a sheet or web of plastic using a gravure printing process, although other coating processes, as known in the art may be used. The primer adheres well to the plastic and the primer coated surface can be printed with liquid toners, such as for example those described in U.S. Pat. No. 5,407,771 to Landa et al., the disclosure of which is incorporated herein by reference. Toners are ElectroInk(copyright) produced and sold by Indigo N. V. of the Netherlands such as ElectroInk(copyright) EI-Mark 3.0 and EI-Mark 3.1 are suitable. Other toners can be used.
There is thus provided, in accordance with an exemplary embodiment of the invention, a method for preparing a plastic surface for printing with a toner comprising:
forming a layer of an aqueous mixture comprising a first material and a second material on the surface, wherein the first material has an affinity for the plastic and the second material has an affinity for the first material and the toner; and
drying the layer,
wherein, when dry, the layer has a weight per square meter of less than 1.5 grams.
Optionally, the weight per square meter is greater than about 0.1 gram, less than about 1 gram, between 0.1 and 0.2 grams or about 0.15 grams.
Optionally, the first material is polyethylenimine.
Optionally the second material is a material chosen from the group of materials comprising: polyethylene acrylic acid copolymer, styrene acrylate copolymer, styrene-butadiene and a salt of an acidic polymer.
Optionally, the weight of the first material plus the weight of the second material is between about 2% and about 10% of the weight of the mixture.
Optionally, the ratio of the weight of the second material to the weight of first material in the mixture is between 1 and 5. Optionally it is between 1 and 2 or between 2 and 3.
Optionally, second material is a neutralized acid polymer material.
There is further provided, in accordance with a preferred embodiment of the invention, a method for preparing a plastic surface for printing with a toner comprising:
forming a layer of an aqueous mixture comprising a first material and a second material on the surface, wherein the first material has an affinity for the plastic and the second material has an affinity for the first material and the toner; and
drying the layer,
wherein the second material is a neutralized acid polymer material.
Optionally, the acid polymer material is polyethylene acrylic acid copolymer.
In some embodiments of the invention, the method includes adding a surfactant to the mixture.
There is further provided, in accordance with an embodiment of the invention, a method for preparing a plastic surface for printing with a toner comprising:
forming a layer of an aqueous mixture comprising a first material and a second material on the surface, wherein the first material has an affinity for the plastic and the second material has an affinity for the first material and the toner and a surfactant in an amount by weight of the mixture of between 0.5 and 2.5%; and
drying the layer,
Optionally, the surfactant is chosen from the group of materials comprising: 2,4,7,9-tetramethyl-5-decyn-4,7-diol; 2,4,7,9-tetramethyl-5-decyn-4,7-diol+3.5 moles ethylene oxide; and sodium dodecyl sulfate.
When the surfactant is 2,4,7,9-tetramethyl-5-decyn-4,7-diol or 2,4,7,9-tetramethyl-5-decyn-4,7-diol+3.5 moles ethylene oxide, optionally, after addition of the surfactant, the mixture comprises between 1% and 1.5% by weight surfactant.
When the surfactant is sodium dodecyl sulfate, optionally, after addition of the surfactant, the mixture comprises between 2% and 2.5% by weight surfactant.
In an exemplary embodiment of the invention, the first material is dissolved in water. Optionally, the aqueous solution of the first material comprises between 2% and 10% by weight of the first material.
In an exemplary embodiment of the invention, the second material is dispersed in water. Optionally, the aqueous dispersion of the second material comprises between 2% and 10% by weight of the second material.
In an embodiment of the invention, forming the layer comprises preparing the mixture in accordance with a procedure comprising:
preparing an aqueous solution of the first material;
preparing an aqueous dispersion of the second material; and
mixing the aqueous solution and the aqueous dispersion.
Optionally, forming the layer comprises forming a layer of an aqueous mixture that does not release organic compounds into the air during drying thereof.
Optionally, forming the layer of the mixture on the surface comprises applying the mixture to the surface using a gravure roller.